A number of prior art devices arranged in the track area are used to detect or locate an overheated wheel bearing. This type of a device is disclosed, for example, in DE-OS No. 29 07 945. In devices of this type, cooled detectors are used as heat radiation sensors.
In addition to the thermal detectors (such as bolometers) that are conventionally used as the detector, a group of rapidly responding heat radiation sensors exists, such as HgCd:HgTe, InSb, PbSe or combinations of these types of semiconductors. Semiconductor detectors of this type respond to changes by means of the thermal excitation of free charge carriers and are able to resolve a radiation of a high impulse sequence, but are not suitable for the continuous detection of a certain temperature level without additional devices, such modulators or deflecting devices that cyclically interrupt the incident ray or guide to other temperature levels.
Devices of this type are usually arranged in the track area, and the measuring ray reaches the generally cooled detector either vertically or in a direction deviating from the vertical line through a window of the device and corresponding deflection devices.
For a better calibration of devices of this type, it was suggested that the respective signal of the unknown source be compared with a reference source. Thus, for example, DE-OS No. 23 43 904 shows the aforementioned type of device wherein a reference source is housed in a pivotable lid which, after the passing-through of all wheels, could be swung into the path of the rays and in this way provided the detector with an additional reference signal. In the case of this known device, the standard emitter, in the waiting position of the system, was located in the path of the rays, whereas, during the measuring time, the signals of the standard emitter could not be taken into account because the lid that carried the standard emitter had to be swung to the side for the measuring.
Based on U.S. Pat. No. 2,978,859, it has become known to provide the detector intermittently with a standard emission along with an emission from an unknown source. In the case of this known device, a rotating disk-shaped modulator is provided, the axis of rotation of which is arranged in a sloped way with respect to the axis of the rays of an unknown emitter and to the axis of the rays of a standard emitter. In the case of an arrangement of this type, when a slotted disk is used, the temperature of the measuring point can always be detected when the slot exposes the path of the rays to the measuring point, and, when a wing of the modulator disk covers this path of the rays, a standard radiation source can be guided to the detector by reflection.
However, in every case, the known devices have always detected only one certain measuring point, and a temperature profile over a preferred direction of the measuring distance could not be measured in any way.